Abstract

This manuscript discusses the materials physics of thermal and electrical transport in the solid state. In particular, the focus is on thermoelectric materials, which enable the direct conversion between thermal and electrical energy. The ability of simple approximations and semiclassical models to describe transport is explored in a variety of systems. In some cases, the traditional models provide a very accurate description of the transport for the compositions of interest to thermoelectric applications. This is the case for n-type Ba8Ga16-xGe30+x, where a single, parabolic band model captures the electrical transport and thus allows the accurate prediction of optimal composition for energy conversion. This is not found to be true in La3-xTe4, and more than one parabolic conduction band is required to describe the electrical transport. In this case, the use of ab initio electronic band structure calculations provided critical knowledge for physical models to be developed. The influence of structure on thermal transport is also examined in detail. The compounds considered typically possess low lattice thermal conductivity, with values often being less than or equal to 1 W/m/K at 300 K. This can generally be associated with large unit cells, where the high number of atoms per unit cell results in a large number of optical modes, which carry little heat due to their low group velocities. Phonon scattering is also considered, and the cation vacancies in La3-xTe4 are found to reduce the lattice thermal conductivity by over 100% at room temperature. Finally, the resulting thermoelectric efficiency is discussed, where leg efficiencies near 20% of the Carnot efficiency are predicted in segmented legs. The work detailed here has led to the continued development of La3-xTe4 by the Jet Propulsion Laboratory, where it is a top candidate for future use in deep-space power-generation systems.